1. Field of the Invention
The present invention relates to a CDMA (code division multiple access) transmitter, which is advantageously applicable to a CDMA system according to the TIA/EIA/IS-95 standard predominant in North America.
2. Description of the Background Art
The total number of subscribers of the cellular system in the United States reached 34,000,000 at the end of 1995, and is expected to keep a double figure growth rate every year. Accordingly, it is expected that the analog systems cannot meet the demand, and hence digital cellular systems with superior frequency availability have been investigated to be carried out in practice. Since Qualcomm Incorporated has disclosed the CDMA system, it has attracted a great deal of attention as a system achieving large capacity. In this respect, reference is made to M. Leonar, "Digital Cellular System in the U.S., CDMA and TDMA are competing", Nikkei Electronics, January 1993, Nikkei BP Inc., Tokyo. The CDMA system spreads the frequency bandwidth of a transmitted signal to 1.25 MHz using a spread spectrum scheme, and carries out fine transmission power control. It is expected to implement a large capacity system by employing diversity effects and soft handoff.
Thus, portable telephone manufacturers have been developing the CDMA cellular systems to meet the demand of the digital cellular. The portable telephone systems under development now are CDMA/AMPS dual mode cellular systems in accordance with the TIA/EIA/IS-95 standard in the U.S., considering that the analog cellular systems (AMPS) and CDMA cellular systems will coexist for some time from now on. In this respect, reference is also made to "TIA/EIA/INTERIM STANDARD: Mobile station--base station compatibility standard for dual-mode wideband spread spectrum cellular system" TIA/EIA/IS-95, July 1993.
The CDMA transmitters of the conventional systems, however, have a problem in that they must meet severe demands on components like a transmitting mixer, such as low noise figure NF, high gain G and low nonlinear distortion ratio k to satisfy the standard. This presents a problem in the selection and development of the components, and has a great influence on the cost and characteristics like dissipated current of the system.